System and method for detection of zero current condition

ABSTRACT

A method for converting power includes charging an inductor by coupling the inductor to a voltage source for a predetermined amount of time. Thereafter, the inductor is discharged by coupling the inductor to a ground until the current flowing through the inductor equals zero. A method for detecting a zero current flowing through the inductor includes coupling the inductor to a transistor and comparing the output of that transistor to a transistor coupled to ground.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of application Ser. No.09/978,125, filed Oct. 15, 2001, which will issue as U.S. Pat. No.6,507,175 on Jan. 14, 2003, and which claims the benefit of provisionalpatent application Ser. No. 60/240,340, filed Oct. 13, 2000.

FIELD OF INVENTION

[0002] This application relates generally to electronic circuits andmore particularly to an electronic circuit for detecting a zero currentcondition, where such a circuit can be used in voltage regulators andswitching power converters (“SPC”), including multiphase powerconverters.

BACKGROUND OF THE INVENTION

[0003] Power regulators are often used in electronic equipment to supplypower at a predetermined voltage to a system. For example, a typicaldesktop computer may contain a power supply that converts alternatingcurrent (“AC”) from a wall socket, to direct current (“DC”) with avoltage that is usable by the various components of the computer system.With continued reference to computer systems, a hard disk drive mayrequire a 12 volt (“V”) power input, while various integrated circuitcomponents may require, for example, power at 5.0 V, 3.3 V, or 1.5 V. Apower supply must thus contain power regulators to generate the requiredvoltage levels.

[0004] Buck power regulators are often used to generate power outputsfor microelectronic devices because they are relatively efficient andprovide high current slewing (di/dt) capability. When providing amicroprocessor with a regulated input voltage, di/dt and response timeare very important considerations. The output inductor value of theregulator determines the di/dt capability of the regulator and also theboundary between continuous conduction mode (“CCM”) (when the inductorcurrent is continuous) and discontinuous conduction mode (“DCM”) (whenthe inductor current is not continuous, but drops to zero until thetransistor is turned ON; DCM typically occurs when a low load resistanceis coupled to the buck power regulator.)

[0005] With reference to FIG. 1, an exemplary buck (step-down) powerregulator 100, which converts a DC voltage to a lower voltage, ispresented. A supply voltage, V_(s), is input into transistor 102, whichis coupled to a diode 104 that, in turn, is coupled to ground. Coupledto the junction of transistor 102 and diode 104 is an LC circuitcomprising an inductor 106 and a capacitor 108. A load 1 10 thusreceives power at the required voltage, where the voltage is determinedby the duty cycle of transistor 102 (i.e., the percentage of time whentransistor 102 is turned on).

[0006] When transistor 102 is on, inductor 106 is being charged and thesupply voltage supplies the output current. When transistor 102 isturned off, inductor 106 “freewheels” through diode 104 and supplies theenergy to load 110. The purpose of the diode is not to rectify, but tore-direct current flow in the circuit and to ensure that there is a pathfor the current from the inductor to flow. Capacitor 108 serves toreduce the ripple content in the voltage, while inductor 106 smoothesthe current passing through it.

[0007] A problem of the buck power regulator is that, as low voltageoutputs are required, the voltage drop of diode 104 leads to variousconsequences. For example, the circuit becomes less efficient because ofthe voltage drop of approximately 0.7 volt across the diode. Suchinefficiencies become less tolerable when devices run on battery poweras opposed to AC power.

[0008] In response to the above deficiencies, buck power regulator 200,detailed in FIG. 2, was developed. As can be seen, buck power regulator200 is similar to buck power regulator 100, with a transistor 204replacing diode 104. Transistor 204 may be configured to have a low onresistance. Transistor 102 is usually termed the high-side switch andtransistor 204 is the low-side switch. In addition, drivers 222 and 224control the operation of transistors 102 and 104, respectively. Bycontrolling the on and off cycles of transistors 102 and 204, drivers222 and 224 are able to more efficiently control the output voltage,V_(out), that is present at load 110, and supply the desired amount ofcurrent.

[0009] In normal operation of a power converter, there is a ripple inthe output current, due to the charging and discharging of inductor 106.One method of reducing the ripple of the output current is the use of amultiphase power supply. Instead of having, for example, a single sourcesupplying a 20 amp output, there may be four phases, each of whichsupply 5 amps. An exemplary multiphase buck power converter is shown inFIG. 12.

[0010] In multiphase power converter 1200, it is desired to convert aninput voltage at terminal 1202 to an output voltage at terminal 1204across a load 1206. In a manner similar to that described above withrespect to FIG. 2, transistors 1212 and 1214 are each coupled to theinput voltage 1202. Coupled to the junction 1211 of transistors 1212 and1214 is inductor 1216. Similarly, transistors 1222 and 1224 are eachcoupled to the input voltage 1202. Coupled to the junction 1221 oftransistors 1222 and 1224 is inductor 1226. Similarly, transistors 1232and 1234 are each coupled to the input voltage 1202. Coupled to thejunction 1231 of transistors 1232 and 1234 is inductor 1236. Similarly,transistors 1242 and 1244 are each coupled to the input voltage 1202.Coupled to the junction 1241 of transistors 1242 and 1244 is inductor1246. Each of the transistor pairs is coupled to capacitor 1208 toprovide the output needed at output 1204. Because of the presence offour power converters, each converter is only responsible for one-fourthof the total current needed, resulting in smaller transistors andinductors and a corresponding reduction in cost. In addition, the ripplein the output current is reduced because each of the converters is onlyresponsible for a portion of the output current. The phases are slightlyoffset from each other such that the peak current of each individualphase do not coincide with each other. This is shown in FIG. 15, whichshows the individual output currents for each phase as well as the totaloutput current. As can be readily seen, the ripple in the output currentis substantially reduced from the ripple in the current of eachindividual phase, and the period of the ripple is approximatelyone-fourth of the ripple of each individual phase.

[0011]FIG. 3 presents a plot of the inductor current of an exemplarybuck power regulator. Axis 302 represents the passage of time, whileaxis 304 details the current flowing through inductor 106. The currentflowing through inductor 106 rises for the time period Ton whentransistor 102 is on and the current falls during time period T_(off),when transistor 102 is off. The period, T, is T_(on) plus T_(off). Theoutput voltage would be the input voltage times T_(on).

[0012] Problems may arise, however, when buck power regulator 200 isrequired to produce a voltage through a smaller load. An exemplaryresulting current plot is shown in FIG. 4. It can be seen that thecurrent through inductor 106 becomes negative during a portion of thecycle, i.e., the current through inductor 106 reverses direction andflows into the ground. This behavior is undesirable because of thevarious inefficiencies that occur because the inductor is basicallywasting power that would ideally remain in the system. Such a problemmay not be present in buck power regulator 100 of FIG. 1, as diode 104automatically “turns off” when the polarity of the inductor currentchanges.

[0013] It is desirable to develop a method and apparatus for convertingvoltage that alleviate the above and other problems that may be presentin the prior art.

SUMMARY OF THE INVENTION

[0014] The present invention uses a Zero Current Detection (“ZCD”)circuit to determine the direction of current flow in various circuits,such as a switch of a switching power converter (“SPC”). In such amanner, once zero current is detected, the operation of the circuit canbe changed such that inefficiencies are reduced.

[0015] In one embodiment, the ZCD circuit may comprise a pair of currentmirrors supplying current to a matched pair of transistors. One of thetransistors is coupled to ground while the other transistor is coupledto the node of interest. The outputs of the matched pair are input intoa comparator. When the non-inverting input voltage exceeds the invertinginput voltage, the comparator changes state.

[0016] In one embodiment, the ZCD circuit may be used in a SPC that isconfigured as a buck converter having Field Effect Transistors (“FETs”)used as power switches. The ZCD signal may be used to maximize theefficiency of the system by controlling the operation of the FETs duringDCM operation. In such a manner, the current flow through the inductoris prevented from becoming negative.

[0017] In another embodiment, the ZCD circuit may be used in amultiphase power converter in a tri-state mode to decrease the switchingtime when transients occur.

[0018] The result is increased system efficiency and faster transientresponse.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] A more complete understanding of the present invention may bederived by referring to the detailed description and claims whenconsidered in connection with the Figures, where like reference numbersrefer to similar elements throughout the Figures, and:

[0020]FIG. 1 illustrates an exemplary buck power regulator of the priorart;

[0021]FIG. 2 illustrates an improved buck power regulator;

[0022]FIG. 3 shows the inductor current of an exemplary buck powerregulator supplying a high voltage;

[0023]FIG. 4 shows the inductor current of an exemplary buck powerregulator supplying a low voltage;

[0024]FIG. 5 presents a further improvement to a buck power regulator;

[0025]FIG. 6 illustrates an exemplary embodiment of the zero currentdetector;

[0026]FIG. 7 shows a voltage/time curve of an embodiment of the presentinvention;

[0027]FIG. 8 shows an alternative embodiment of a buck power regulatorusing the zero current detector;

[0028]FIG. 9 shows the output current when a transient occurs;

[0029]FIGS. 10 and 11 shows the operation of the switches in a buckpower regulator;

[0030]FIG. 12 illustrates an exemplary multiphase buck power converter;

[0031]FIG. 13 illustrates an exemplary multiphase buck power converterwith a zero current detector;

[0032]FIG. 14 shows the output voltage present during a load transient;

[0033]FIG. 15 shows the output current when using a multiphase powerconverter;

[0034] and

[0035]FIG. 16 shows the output current when using a multiphase powerconverter using a zero current detector.

DETAILED DESCRIPTION

[0036] The present invention may be described herein in terms of variousfunctional components and various processing steps. It should beappreciated that such functional components may be realized by anynumber of hardware or structural components configured to perform thespecified functions. For example, the present invention may employvarious integrated components comprised of various electrical devices,e.g., resistors, transistors, capacitors, diodes, inductors, and thelike, whose values may be suitably configured for various intendedpurposes. In addition, the present invention may be practiced in anyintegrated circuit application where a detection of a zero current flowis desired. Such general applications that may be appreciated by thoseskilled in the art in light of the present disclosure are not describedin detail herein. Further, it should be noted that while variouscomponents may be suitably coupled or connected to other componentswithin exemplary circuits, such connections and couplings can berealized by direct connection between components, or by connectionthrough other components and devices located therebetween.

[0037]FIG. 5 presents an exemplary embodiment of a buck power regulator.A transistor 502 is coupled to a transistor 504 at node 511. Coupled tothe junction 511 of transistors 502 and 504 are inductor 506, capacitor508, and load 510. A driver 522 is coupled to transistor 502 and adriver 524 is coupled to transistor 504. Drivers 522 and 524 serve tocontrol when transistors 502 and 504, respectively, are conducting andwhen they are off. In addition, there is also a zero current detector512 coupled to junction 511. The output of zero current detector 512 iscoupled to a controller 514, which is coupled to both drivers 522 and524 to control the conduction of transistors 502 and 504. The output ofthe regulator is at node 520.

[0038] With reference to FIG. 4 and FIG. 5, it can be seen that thecurrent through inductor 506 is identical to the current at junction511. When the current at junction 511 reaches a level of zero amps,current detector 512 outputs a signal to controller 514, which thensends a signal to driver 524, which turns transistor 504 off. Withtransistor 504 turned off, current no longer flows from inductor 506into ground. The energy in the inductor also becomes zero and capacitor508 cannot discharge into ground because switch 504 is closed, resultingin a more efficient power regulation than a buck power regulator with adiode.

[0039] With reference to FIG. 6, an exemplary zero current detectioncircuit 600 is disclosed. A current source 602 supplies current totransistor 604. Transistors 606 and 610 act as a current mirror andsupply substantially identical current to transistors 608 and 612, eachof which may be configured in diode-connected fashion, as illustrated inFIG. 6. In a preferred embodiment, transistors 606 and 610 are matchedto within a tight tolerance of each other. Transistors 608 and 612 arealso matched with to within a tight tolerance of each other. The sourceof transistor 608 is coupled to ground, while the source of transistor612 is coupled to junction 511 (see FIG. 5). Both transistors 608 and612 are coupled to inputs of comparator 620. Comparator 620 is coupledto controller 514. Comparator 620 is configured such that, when bothinputs to comparator 620 are equal, the output of comparator 620changes, as detailed in FIG. 8. Comparator 620 is preferably alow-offset, high-speed comparator.

[0040] Both transistor 608 and transistor 612 are coupled to the sameamount of current, via current mirrors 606 and 610, respectively.Because the source of transistor 608 is coupled to ground and the sourceof transistor 612 is coupled to junction 511, transistors 608 and 612produce an equal voltage at their respective drains when the input tothe source of transistor 608 is equal to the input of the source oftransistor 612. In other words, when junction 511 is equal to ground(i.e., the voltage is zero), the voltages at the drains of transistors608 and 612 are equal. Thus, transistors 608 and 612 serve to levelshift the inputs into the common-mode range of comparator 620. It can bereadily shown that the voltage at junction 511 is zero only when thereis no current flowing through junction 511. Transistors 614 and 618serve to prevent an excessive voltage level at comparator 620 bydirecting excessive voltage to ground.

[0041] The voltage at junction 511 is ideally shown in FIG. 7. It can beseen that the voltage at junction 511 is at a peak when transistor 502first turns on and inductor 506 is being charged by the power supply.The voltage at junction 511 drops below zero voltage when transistor 502turns off, as inductor 506 pulls charge from ground through transistor504, resulting in a negative voltage potential at junction 511. Thevoltage reaches zero when the current through inductor 506 begins toflow in the opposite direction, from inductor 506, through transistor504, to ground. Thus, it can be seen that, by sensing the voltage atjunction 511, the zero current detect circuit is able to determine whenthe current is zero by detecting when the voltage at junction 511 iszero.

[0042] Thus, zero current detection circuit 600 disclosed in FIG. 6 canbe used in place of element 512 of FIG. 5 to detect when the currentthrough inductor 506 begins to flow in the negative direction. Once itis determined that a zero current condition is present, driver 524 canbe configured to turn off transistor 504 to prevent voltage from flowingfrom inductor 506 to ground. This results in increased efficiency as theamount of energy lost to ground is drastically reduced.

[0043] An alternative embodiment of the power regulator is presented inFIG. 8. Power regulator 800 features a voltage source 830 that feeds aswitch 802. It should be understood that switches 802 and 804 maysuitably be replaced with a transistor switch and diode 814 is shownparallel to switch 804 to demonstrate an FET switch. A load inductance832 and a capacitor 808 and a load 810 is also present in the circuit.Also illustrated in FIG. 8 are parasitic inductances 836, 834, and 838.The output of the circuit can be taken at node 820.

[0044] Voltage is sensed at both sides of load 810. The measurementtaken at the high side of the line, at node 840, is termed V_(sense)+.The measurement taken on the low side, from node 842, is termedV_(sense)−. The two voltage measurements are input to controller 812,which operates switches 802 and 804. The two voltage measurements serveto provide a more accurate reading, to controller 812, of when a changein the load is encountered. It should be understood that a zero detectcircuit, although not illustrated, may also be present in powerregulator 800. Such a zero detect circuit may be coupled to node 811 tosense a zero current condition. The presence of the zero currentcondition can be forwarded to controller 812 to more accurately controlswitches 802 and 804.

[0045] The operation of the circuit may be described more fully withrespect to FIGS. 9-11. FIG. 9 illustrates a graph of the current throughload 810 in exemplary operation. As can be seen at the left end of thegraph, when load 810 presents a low load (high impedance), the currentthrough load 810 is also low. However, when the impedance is decreased,current through load 810 rises to a high value, as can be seen at theright end of FIG. 9. The time period during the transient from thesteady-state operation at low load and the steady-state operation athigh load is depicted as region 902 and may be termed the hystereticmode.

[0046] During the steady-state modes, the operation of switches 802 and804 are periodic, as depicted in FIG. 10 for switch 802 and FIG. 11 forswitch 804. During those periods, switch 802 and 804 may operate in amutually exclusive manner, as shown in FIGS. 10 and 11. In other words,when switch 802 is on, switch 804 is off and when switch 802 is off,switch 804 is on. The ratio between the on time and off time of theswitches determines the output voltage of the regulator. However, duringthe hysteretic mode, switch 802 may pulse on and off to set the currentthrough load 810 to the appropriate level. Once the appropriate currentlevel is established, operation of the switches continues as before.

[0047] The result is that, in a relatively small amount of time, circuit800 is able to react to a change in the load and supply the correctamount of current to the load.

[0048] In a multiphase power converter, with reference to FIG. 13, theconfiguration of the circuit is as follows. In multiphase powerconverter 1300, it is desired to convert an input voltage at node 1302to an output voltage at node 1304 across a load 1306. In a mannersimilar to that described above with respect to FIG. 2, transistors 1312and 1314 are each coupled to the input voltage 1302. Coupled to thejunction 1311 of transistors 1312 and 1314 is inductor 1316 and zerocurrent detector 1315. Similarly, transistors 1322 and 1324 are eachcoupled to the input voltage 1302. Coupled to the junction 1321 oftransistors 1322 and 1324 is inductor 1326 and zero current detector1325. Similarly, transistors 1332 and 1334 are each coupled to the inputvoltage 1302. Coupled to the junction 1331 of transistors 1332 and 1334is inductor 1336 and zero current detector 1335. Similarly, transistors1342 and 1344 are each coupled to the input voltage 1302. Coupled to thejunction 1341 of transistors 1342 and 1344 is inductor 1346 and zerocurrent detector 1345. Each of the transistor pairs is coupled tocapacitor 1308 to provide the output needed at output 1304.

[0049] The use of the zero current detector has a profound effect on theoperation of the power converter. It is understood that, when the loadto a power converter increases, there is a corresponding increase in thecurrent. Typically, when such an increase in the current occurs, thereis a corresponding decrease in the voltage at the load. With referenceto FIG. 14, the load voltage/time curve of an exemplary power converterof the prior art is shown. The voltage begins at a level ofapproximately 1.15 volts. When a load transient occurs and more currentis being drawn from the power converter, the voltage at the loaddecreases to approximately 0.85 volts and remains lower than requiredfor a certain time period, while the power converter is adapting to thechange in current. Once the power converter has adapted, the outputvoltage is back at the specified 1.15 volts. Modern electronics requirea very steady supply voltage in order to operate correctly. Such aprolonged droop in the voltage can be very detrimental to the operationof certain electronic components.

[0050] As described above, the typical configuration of switches in apower converter switches the high side switch and the low side switchsimultaneously, such that only one of the switches is on at one time.During transients, however, there may be an occasion when both switchesare off at one time, with the high side switch pulsing, in order tosupply more current to the load. In addition, as described above, when azero current condition is detected, both switches may be off, to preventcurrent from flowing into ground. Thus, it can be seen that, in order tosupply more current to the load, the low-side transistor (transistors1314, 1324, 1334, and 1344) is turned off.

[0051] One reason for the voltage droop is because, if the low-sideswitch is on, it must be turned off before the current to the load canbe increased. However, with the combination of the zero currentdetection circuit and the multiphase power converter, it can be seenthat there is a greater likelihood of the low-side switches being off,resulting in a faster transient response. With reference to FIG. 16, theoperation of the multiphase power converter with the zero currentdetector will be graphically described.

[0052]FIG. 16 shows the current/time graph of the 4-phase, multiphasepower converter, along with the individual inductor currents. Duringregion 1602 of the graph, one of the individual phases is at zerocurrent, forcing off both the low side and high side switches. Asdescribed above, when the current through an individual inductor isrising, the high-side switch is on and the low-side switch is off. Whenthe current through an individual inductor is falling, the high-sideswitch is off and the low-side switch is on. It can be seen that, duringregion 1602, of the four different phases, only one or two of the otherphases simultaneously have falling inductor current. Therefore, only oneor two low-side switches are on at once. Thus, during a load transient,there is a lesser necessity to turn off low-side switches to meet thehigher current requirements. This results in a faster response totransients due to increased load.

[0053] It should also be understood that such an improved transientresponse time is also present in the embodiment shown in FIG. 5, for thesame reason.

[0054] The above description presents exemplary modes contemplated incarrying out the invention. The techniques described above are, however,susceptible to modifications and alternate constructions from theembodiments shown above. Other variations and modifications of thepresent invention will be apparent to those of ordinary skill in theart, and it is the intent of the appended claims that such variationsand modifications be covered. For example, the present invention may beused in a multiphase power converter with multiple low-side switches.The multiple low-side switches may be switched off (e.g., using atri-state mode of operation) to decrease the response time of theconverter. While the zero current detection circuit was described withrespect to a buck power regulator, it can be used in various otherapplications. For example, the zero current detection circuit can beused in a highly-phased power regulation system such as those used inlow-voltage conversion applications such as for microprocessor loads.The ZCD may be used as part of a power IC to detect when a switchingelement is at ground. When zero current is detected, the operation ofthe power regulation system may be changed to minimize variousinefficiencies that may be present due to high RMS currents.

[0055] Consequently, it is not the intention to limit the invention tothe particular embodiments disclosed. On the contrary, the invention isintended to cover all modifications and alternate constructions fallingwithin the scope of the invention, as expressed in the following claimswhen read in light of the description and drawings. No element describedin this specification is necessary for the practice of the inventionunless expressly described herein as “essential” or “required.”

We claim:
 1. A method of regulating power comprising: charging aninductor with a source voltage for a first time period by coupling saidinductor to said source voltage; discharging an inductor for a secondtime period by coupling said inductor to a ground; disconnecting saidinductor from both said source voltage and said ground for a third timeperiod; wherein said disconnecting step is commenced when a currentflowing through said inductor equals zero amperes.
 2. The method ofclaim 1 wherein said charging step comprises: turning on a first switchcoupling the inductor to the source voltage; and turning off a secondswitch coupling the inductor to ground.
 3. The method of claim 1 whereinsaid discharging step comprises: turning off a first switch coupling theinductor to the source voltage; and turning on a second switch couplingthe inductor to ground.
 4. The method of claim 1 wherein saiddisconnecting step comprises: turning off a first switch coupling theinductor to the source voltage; and turning off a second switch couplingthe inductor to ground.
 5. The method of claim 1 wherein saiddisconnecting step is commenced by a signal transmitted by a zerocurrent detection circuit.
 6. The method of claim 1 wherein saiddisconnecting step comprises: coupling an input of a first device tosaid inductor; connecting an input of a second device to a ground;supplying said first device and said second device with substantiallyequal currents; comparing the output voltage of said first device withthe output voltage of said second device; and transmitting a signal tocommence said disconnecting step.
 7. The method of claim 6 wherein: saidfirst device comprises transistor comprising a gate, source, and adrain; and said second device comprises a transistor comprising a gate,source, and a drain.
 8. The method of claim 6 wherein: said comparingstep utilizes a low-offset, high-speed comparator coupled to said firstdevice and said second device.
 9. An apparatus for converting powercomprising: a first transistor; a second transistor coupled to groundand to said first transistor, forming a junction between said firsttransistor and said second transistor; an inductor coupled to saidjunction between said first transistor and said second transistor; and azero current detection circuit coupled to said junction between saidfirst transistor and said second transistor.
 10. A method for detectinga zero current condition in a power converter comprising: coupling aninput of a first device to an output inductor of said power converter;connecting an input of a second device to a ground; supplying said firstdevice and said second device with substantially equal currents; andcomparing the output voltage of said first device with the outputvoltage of said second device.
 11. The method of claim 10 wherein: saidfirst device comprises transistor comprising a gate, source, and adrain; and said second device comprises a transistor comprising a gate,source, and a drain.
 12. The method of claim 10 wherein: said comparingstep utilizes a low-offset, high-speed comparator coupled to said firstdevice and said second device.